1ACh11 - Ch. 11 Intermolecular Forces, Liquids, and Solids...

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Ch. 11 Intermolecular Forces, Liquids, and Solids Gas, Liquid, Solid Gas Liquid Solid Entropy High Medium Low Intermolecular Forces Negligible Moderate High Spacing Widely spaced Close packed Close packed Motion Active Active Fixed location Entropy is often described as "disorder", but is also described as "possible states". High entropy has a high number of possible states, this makes predictions more difficult, hence the term "disorder". Physical state is a result of a balance between the kinetic energy of the particles and the intermolecular forces. Intermolecular Forces Intermolecular Forces (forces that occur between molecules) occur in the condensed states ( liquids and solids ) Intermolecular forces are also called cohesive forces London Dispersion Forces Nonpolar molecules also have attractive forces between them. These are called London Dispersion Forces. Motion of electrons around an atom or molecule can result in a momentary charge separation. This instantaneous dipole can induce a similar dipole in a neighboring atom resulting in an electrostatic attraction. This attraction is weak; however, it can be significant and is more significant for larger molecules and atoms. Larger atoms and molecules have more electrons and a larger electron "cloud". This results in an increased ability to form instantaneous dipoles (increased polarizability). The strength of intermolecular forces can be seen in physical properties such as boiling points and melting points. Stronger intermolecular forces result in higher melting points and higher boiling points.
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Dipole-Dipole Forces Molecules with a dipole moment have a charge separation, an area of positive charge and an area of negative charge. These charges interact with the charges on adjacent molecules. The molecules tend to orientate themselves to minimize electrostatic repulsion and maximize electrostatic attraction. This results in a net electrostatic attraction between molecules with dipole moments. This is called Dipole-Dipole Attraction . Hydrogen Bonding A special case of Dipole-Dipole Attraction is called hydrogen bonding . This occurs when hydrogen is bound to a highly electronegative atom ( N, O, F ). This results in a particularly strong attractive force because of the large polarity combined with the small size of the hydrogen, which allows the molecules to get closer. Ion-Dipole Forces Ion-dipole forces are the attractive electrostatic forces that form between an ion and polar molecules. The polar molecules line up the partial charge of opposite polarity with the ion. This creates the attractive force. This force is important for the dissolution of ionic compounds in polar solvents such as water. Strength of Intermolecular Forces
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This note was uploaded on 04/17/2008 for the course CHEM 1 taught by Professor Evans during the Fall '06 term at UC Irvine.

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1ACh11 - Ch. 11 Intermolecular Forces, Liquids, and Solids...

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